Nuclear logging includes methods that detect the presence of unstable isotopes or create such isotopes in the vicinity of a borehole. Several nuclear logging techniques are available including natural gamma logs, gamma-gamma logs, and neutron-neutron logs. Natural gamma logs are probably the most common nuclear methods used in groundwater studies.
Environmental engineers use natural gamma logging, in general, to identify lithology and stratigraphic correlation and, in particular, to evaluate the presence, variability, and integrity of clays and shales.
The natural gamma log records the amount of natural gamma radiation emitted by rocks and unconsolidated materials from a borehole. Different formations can be distinguished from different levels of natural radioactivity as shown in Figure 9.14.5. The gamma-emitting radioisotopes normally found in all rocks and unconsolidated materials are potassium-40 and daughter products of the uranium and thorium decay series (Benson 1991). Clays and shales concentrate these heavy radioactive elements through the process of ion exchange and adsorption; therefore, their natural gamma activity is much higher than that of other materials.
The natural gamma log instrumentation is relatively simple and inexpensive and involves radiation detection only. However, only qualitative analysis is possible with this method, and the sensitivity of the probe is reduced by large diameter holes, drilling fluid, and casing (U.S. EPA 1993a).
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